|(Wollheim) - Fantasy whose only believability is in the reader's artificial acceptance of it for the sake of the story. It may take beliefs which were once widely held, such as Hellenic mythology, but if it does it must mix in a modern element, otherwise you're in the providence of weird fiction. Also there may be a gesture at a pseudoscientific or "you can't be sure" explanation, but this doesn't make it science fiction, because the explanation isn't to be taken seriously.
Pure fantasy as a regular form appeared late, aside from fairy tales of children and tales allegedly for children (such as Lewis Carroll's masterpieces). The "modern mythology" of Unknown was for the most part pure fantasy.